[match-case] Fix narrowing of class pattern with union-argument. #19517
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randolf-scholz
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Jul 27, 2025
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| if not proposed_type_ranges: | ||
| # This is the case for `if isinstance(x, ())` which always returns False. | ||
| return UninhabitedType(), default |
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This test is actually redundant, it works just as well if we comment it out since make_simplified_union([]) returns UninhabitedType().
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Diff from mypy_primer, showing the effect of this PR on open source code: prefect (https://github.com/PrefectHQ/prefect)
- src/prefect/tasks.py:559: error: Incompatible types in assignment (expression has type "Union[int, float, list[float], None]", variable has type "list[float]") [assignment]
+ src/prefect/tasks.py:559: error: Incompatible types in assignment (expression has type "Union[float, int, list[float], None]", variable has type "list[float]") [assignment]
hydra-zen (https://github.com/mit-ll-responsible-ai/hydra-zen)
- src/hydra_zen/wrapper/_implementations.py:932: error: Incompatible return value type (got "type[object]", expected "DataClass_ | type[DataClass_] | ListConfig | DictConfig") [return-value]
spark (https://github.com/apache/spark)
- python/pyspark/pandas/series.py:5116: error: Need type annotation for "cond" [var-annotated]
- python/pyspark/pandas/series.py:5129: error: Incompatible types in assignment (expression has type "Column", variable has type "Never") [assignment]
- python/pyspark/pandas/series.py:5131: error: Incompatible types in assignment (expression has type "Column", variable has type "Never") [assignment]
- python/pyspark/pandas/series.py:5134: error: Incompatible types in assignment (expression has type "Column", variable has type "Never") [assignment]
- python/pyspark/ml/connect/readwrite.py:100: error: Redundant cast to "JavaEstimator[Any]" [redundant-cast]
- python/pyspark/ml/connect/readwrite.py:103: error: Redundant cast to "JavaEvaluator" [redundant-cast]
sympy (https://github.com/sympy/sympy)
- sympy/polys/domains/domain.py:542: error: Cannot infer value of type parameter "Es" of "convert_from" of "Domain" [misc]
- sympy/polys/domains/domain.py:549: error: Cannot infer value of type parameter "Es" of "convert_from" of "Domain" [misc]
- sympy/polys/domains/domain.py:551: error: Cannot infer value of type parameter "Es" of "convert_from" of "Domain" [misc]
- sympy/polys/domains/domain.py:584: error: Incompatible types in assignment (expression has type "Basic", variable has type "Es | Expr | complex") [assignment]
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Sibling PR to #19473, with some experimental changes.
conditional_typesfunction.UninhabitedType(), defaultwhen no ranges are given. This corresponds toisinstance(x, ()), with an empty tuple, which always returnsFalseat runtime.testIsinstanceWithOverlappingPromotionTypesrestrict_subtype_away: if the second argument is aTypeVar, replace it with its upper bound (crucial to get correct result intestNarrowSelfType)TypeChecker.get_isinstance_typeto return empty list (fixesisinstance(x, ())behavior).Modified tests
testIsInstanceWithEmtpy2ndArgnow correctly infers unreachable forisinstance(x, ()).testNarrowingUnionMixinsnow predicts the same results as pyright playground, https://mypy-play.net/?mypy=latest&python=3.12&gist=734c95f128705cfb0dc4ce24f1ad8eec